Cow Farts and Beefy Burps

March 16, 2011

Safety is always an important consideration when working in a laboratory. When I was working in industrial research, my employer had a very comprehensive safety program, part of which was the requirement for us to document any unusual experiments and have them vetted by the safety department. In one of my experiments, there was a potential for a small quantity of hydrogen gas to be released. Would this be an explosion hazard? My calculation showed that the maximum likely hydrogen release would be only six STP milliliters per hour.

What does this translate to as an explosion hazard? When hydrogen and oxygen combine, the reaction releases 59 kcal (247 kJ) per mole of H₂ consumed. My six milliliters, collected over the course of an hour, is just 2.5 x 10^-4 moles of H₂. This would release only 14.75 calories, or 61.7 Joule. Since nitrogen gas (the major part of air) has a heat capacity of 29 Joule/mole-K, this reaction will cause a fifty degree temperature rise in the volume, or just a 17% increase in pressure. This doesn't even correct for the initial 6 ml of hydrogen and 3 ml of oxygen consumed in the reaction, which will slightly lower the pressure. I characterized the experiment as having "less explosive potential than a fart."[1]

I've recently posted an article about cows (Geomagnetic Alignment of Cattle, February 11, 2011). People in our modern culture don't think much about cows, but domestication of cattle was an important milestone in the development of civilization. Since I was raised in Upstate New York, where a lot of dairy farming takes place, I have one anecdote about cows.

I would often assist my father in his construction business, and every so often we would be called into a rural area to give an estimate. One farm we visited had a big sign at the front gate with a picture of a cow and the words, "Registered Holsteins," below it. Rural folk know that Holstein is a breed of cow. My father, however, was a city boy, and he addressed our potential customer as "Mrs. Holstein."

Drawing of a Holstein Cow by Pearson Scott Foresman, via Wikimedia Commons.

Just a few days away, March 26, will mark Earth Hour, the one hour of the year in which we're encouraged to turn off the lights to save energy, but mostly to raise our environmental conscience. I've been turning off unnecessary lights and electrical appliances for years, so I don't plan to participate; but the Earth Hour organizers suggest we do other things, beyond the hour. The popular Chinese actress, Li Bingbing, has pledged to adhere to a vegetarian diet for a hundred days.[2]

The reason cited for Bingbing's pledge is the energy cost and CO₂ emissions associated with meat production and distribution. It's estimated that it takes about thirteen pounds of grain to produce one pound of beef.[3] Furthermore, land planted with cereals will produce five times more protein than grazing land for meat production.[3] As if these reasons are not enough, we need to consider cow fart-induced global warming.

According to a 2006 research report by the United Nations,[4] cattle farming is responsible for 18% of greenhouse gases, which is more than that generated in toto by transportation. Wrote the senior author of this report,[4]

Livestock are one of the most significant contributors to today’s most serious environmental problems. Urgent action is required to remedy the situation."

Cattle emit large quantities of methane because of the methanogenic bacteria in their digestive system; but most of the gas is expelled through burping, not flatulence. It breaks down into 95% beefy burps, and just 5% cow farts. Although farm machinery generates CO₂, methane is about twenty times more potent a greenhouse gas than carbon dioxide. This is because blackbody radiation for objects near room temperature (i.e., the Earth) peaks at about a 10 μm wavelength, That's near where methane does most of its absorption, as shown in the figure.[5]

Infrared spectra for carbon dioxide and methane. Methane absorbs more strongly in the radiation range for a room temperature blackbody. (Data from NIST).[5]

Estimates are nice, but scientists demand real data. That's why there have been continued efforts to pin down exact numbers for methane emission by cattle. A recent study in the Journal of Environmental Quality reviews the cattle methane problem, and it describes instrumentation for accurate measurements.[6-7] Such accurate measurement is important to determine whether changes in the bovine diet can lead to reduced methane emission. Instead of measurements from isolated cattle, this new research used open path lasers to assess methane emission from a herd of eighteen cattle in a paddock. The measurement yielded a methane production that was about 140 g/animal/day.

Using more expensive feed to control the methane released by cattle might even pay for itself. Methane production can reduce the energy content of feed by up to 12%, which is less energy available to produce meat and milk.

It might not be as exciting as sharks with laser beams attached to their heads, but the lead author of the paper says that the laser technique is a "significant advancement in assessing greenhouse gas emissions from the cattle industry."[6]

References:

1. Disclaimer - Your experiment may not conform to my model and my calculation. No warranty of correctness is either given or implied. In other words, don't take advice from strangers.
2. Earth Hour: from switching off to taking action, Agence France-Presse, March 2, 2011.
3. Book Review of Eating by Peter Singer and Jim Mason; Peter Singer interview, BBC News, September 4, 2011.
4. Christopher Matthews, "Livestock a major threat to environment-Remedies urgently needed," Food and Agriculture Organization of the United Nations (Rome, November 26, 2006).
5. NIST Chemistry WebBook, National Institute of Standards and Technology, Standard Reference Data Program.
6. Sara Uttech, "Measuring Methane-Researchers Develops Technique to Measure Methane Gas from Cattle," Press Release, American Society of Agronomy.
7. S. M. McGinn, D. Turner, N. Tomkins, E. Charmley, G. Bishop-Hurley and D. Chen, "Methane Emissions from Grazing Cattle Using Point-Source Dispersion," Journal of Environmental Quality, vol. 40, no. 1 (January 2011), pp. 22-27.